The present invention relates to an optical coupler for applying a near-infrared beam from a light-emitting element to an optical fiber with high efficiency in an optical transmitter used for optical fiber communications.
For data transmission using an optical fiber, in order to improve the transmission distance and the transmission characteristics of the fiber, it is essential to increase the amount of optical power in the optical fiber. Heretofore, a lens of optical glass has been used to transmit a light beam from a light-emitting element to an optical fiber. However, such a lens has a large aberration, and therefore the quantity of light entering the core of the optical fiber is considerably small.
FIG. 1 shows a laser-diode coupler of a type described in a paper by Y. Tachikawa et al. read at the IOOC 4th international Conference 30C2-1 (1983-6-30). In FIG. 1, reference numeral 1 designates an LD (laser diode); 2 a sapphire spherical lens; 3, an optical fiber; and 4, a cap which hermetically seals the LD 1 and holds the sapphire spherical lens 2.
FIG. 2 is a diagram for a description of a method of calculating the coupling efficiency of an optical coupler. In polar coordinates with the light-emitting center of the LD as the origin point, the angle of rotation around the optical axis is represented by .psi., and the angle with the optical axis by .theta.. The light-emitting area of the LD is much smaller than the diameter of the lens 6 and the fiber 7, and therefore the LD can be approximated as a point light source. Thus, the coupling efficiency .eta. between the LD and the fiber is as follows: ##EQU1## where W(.theta.,.psi.) is the power density per unit solid angle extending in a direction (.theta.,.psi.) from the LD, and A.sub.c (.theta.,.psi.) is the fraction of the light radiated in that direction coupled through the lens to the fiber. A.sub.c (.theta.,.psi.) can be obtained by a ray tracing method whereby the position and the direction of a light beam from the LD are obtained when the light beam enters the end face of the fiber, and these values are compared with the amount of light actually carried by the fiber.
If the distance between the incident point of the light beam to the fiber and the center of the fiber core is represented by .gamma..sub.in, the radius of the core by .gamma..sub.c, the incident angle to the fiber by .theta..sub.local, then A.sub.c (.theta.,.psi.) can be represented as follows: ##EQU2## In this connection, .theta..sub.local can be expressed by using the maximum light receiving angle .theta..sub.0 at the core center as follows: ##EQU3##
In general, in a rotationally symmetric single lens coupling system, the following effective angles .theta..sub.eff are inherent to combinations of coupling optical systems and optical fibers: EQU .theta..ltoreq..theta..sub.eff A.sub.c (.theta.,.psi.)=1 EQU .theta.&gt;.theta..sub.eff A.sub.c (.theta.,.psi.)=0 (4)
Expression (1) above can be rewritten as the following expression (5) using expression (4): ##EQU4##
Expression (4) indicates that only light energy radiated within the effective angle from the LD is coupled to the optical fiber. The angle .theta..sub.eff will be referred to as "an effective NA", when applicable. It goes without saying that the larger the effective NA of a coupling optical system coupled to a fiber, the larger the LD-to-fiber coupling efficiency.
With reference to the example shown in FIG. 1, the effective NA will be obtained using a known technique. In accordance with this technique, the diameter of the spherical lens is 2 mm, the refractive index is about 1.76 (for sapphire), and the core diameter of the optical fiber is 40 .mu.m. FIGS. 3 and 4 are diagrams provided for a description of the incidence of a light beam from the LD to the optical fiber with the coupling system arranged most suitably. More specifically, FIG. 3 is a ray tracing diagram showing a light beam from the LD at the coordinate origin point (0,0) which is refracted by a sapphire spherical lens 8 and is applied to the fiber, and FIG. 4 shows an aberration curve. In FIG. 4, with the radiation angle .theta. of a light beam emitted by the LD plotted on the y-coordinate axis and the distance .gamma..sub.in between the incident point of the light beam on the end face of the fiber and the optical axis on the x-coordinate axis, the relation between the .theta. and .gamma..sub.in of the light beam is indicated. In the case of a light beam coupled to a fiber having a core diameter of 40 .mu.m, .gamma..sub.in .ltoreq.20 .mu.m should be established. As is apparent from FIG. 4, light beams satisfying this requirement are such that .theta..ltoreq.16.1.degree.. Expression (2) indicates that factors determining the effective angle .theta..sub.eff reside in both the position of the incident point and the incident angle of the light beam applied to the fiber. Both must satisfy conditions of incidence to the fiber. FIG. 4 illustrates only the condition as to the position of the incident point. In general, in an optical system in which the distance between the lens and the fiber is several times the distance between the LD and the lens, that is, in an optical system having a large magnification factor, the effective angle .theta..sub.eff can be determined only by the condition of the position of the incident point. Accordingly, in the coupling system of the known technique illustrated in FIG. 1, the effective angle .theta..sub.eff is 16.1.degree., and of those rays from the LD, only the rays emitted within the effective angle of 16.1.degree. are coupled to the optical fiber. In such a coupling system, the light beam from the LD is not always effectively coupled to the fiber.